You may be right, that she has the right to know. But I'm unsure if the law gives her that right IF the other party doesn't want to share. I know it used to be that way, maybe it's changed.
Not every mother/child reunion is a happy one. Sometimes mothers adopt out their infants out of love for them, wanting them to have a better life than they can provide. Sometimes the mother is just an utter failure in life, an awful being, who got "knocked up" and couldn't wait to give birth, give away the child and get back her life. In these cases it might be healthier for the child to not know about the parents.
And some adopted children don't want to know. They are happy with their family and their life and never develop the "need to know".
I think in a case as horrendous as this, it might be kinder to the child to keep this potentially life-changing (damaging) information from her, even if she thinks she wants to know. The information is far more traumatic than most adoptions.
But, maybe I'm wrong. Maybe there are some adopted people on this board who might chime in on this.